One method of producing glass fibers is to pass molten glass through openings in a precious metal bushing and attenuate the resulting molten streams of glass into fibers. The metal bushing forms a container which is filled with molten glass. The bottom of the bushing defines a plurality of apertures through which the molten glass is drawn by mechanical means. It is advantageous to heat the bushing to a uniform temperature in order to facilitate and ensure the production of uniform glass fibers. A preferable method of heating the bushing is to pass a high electrical current through it.
The diameter of the fibers produced is dependent upon the composition of the glass, the temperature of the glass, the temperature of the bushing, the thermal conditions below the bushing which affect the rate of cooling of the molten glass fibers and the stress introduced into the fibers by mechanical attenuation thereof. The object of the process is to produce a plurality of glass fibers of uniform diameter which in turn produce uniform package weights. Commonly, bushings having multiple sections are utilized and the maintenance of constant and uniform temperature across each section of such a multiple section bushing has been found to be an important consideration in the production of uniform fiber diameters.
Various schemes have been suggested for controlling the application of heat to individual sections of a multiple section bushing in order that each section operate constantly at a common temperature. For example, U.S. patent application Ser. No. 839,676, filed Mar. 14, 1986, now U.S. Pat. No. 4,657,572 granted Apr. 14, 1987 and owned by the assignee herein balances temperature in a multiple section bushing by diverting current flow from a bushing section which is operating at a temperature above the set point in order to achieve and maintain the set point temperature. In this system, temperature sensing of the individual bushing sections is achieved by sensing the resistance change of the bushing sections and calculating the temperature and temperature deviation from the set point.
Another means of temperature sensing is disclosed in U.S. Pat. No. 4,594,087. Here, a plurality of thermocouples are positioned in various locations along a bushing and thus provide an average temperature reading. A pair of thyristors controllably shunt current flow through each half of the bushing to maintain the desired temperature.
U.S. Pat. No. 4,024,336 discloses a split bushing controller somewhat similar to the above-noted apparatus. Here, two temperature sensing components are utilized. The first drives a first controller which regulates the power supplied to the entire bushing whereas the second temperature sensor and a second controller regulate the relative current to the two bushing sections by controlling a pair of full wave variable impedance devices shunting the bushing halves.
In U.S. Pat. No. 4,546,485, a plurality of thermocouples disposed along a bushing provide an average temperature which is utilized to control and maintain the current flow and thus temperature of the bushing. A manually adjustable variable impedance device may be adjusted to control the relative temperatures of the halves of the bushing in order to achieve and maintain equal throughput.
One of the first considerations to be faced in any glass fiber bushing temperature controller device is the choice of temperature sensing means. Those generally recognized as having utility in this application are thermocouples, infrared, i.e., non-contact temperature measurement and resistance measurement.
Each of the foregoing temperature measurement means is accompanied by advantages and disadvantages. For example, present thermocouple technology provides extremely accurate temperature measurement. However, at the operating temperature of glass fiber forming bushings, that is, around 2,500.degree. Fahrenheit, thermocouples have a relatively short life. Furthermore, they measure temperature only at one point and since they are generally secured to the outside of the bushing, a finite time lag exists between a change in temperature of the molten glass and a change in bushing temperature and the sensing of same by the thermocouple. Infrared temperature measurement techniques though accurate have been frustrated by the presence of the issuing streams of molten glass and the crowded conditions beneath the bushing due to fin shields and other temperature control devices.
Temperature sensing and control through resistance measurement is perhaps the approach best suited to this application but it too is not without obstacles. For example, since the system senses the resistance of the bushing while current is flowing through it, the system is susceptible to noise in the power line and other locally generated interference. Furthermore, presuming the control system adjusts the current flowing through portions of the total bushing, steps must be taken to prevent the adjusted current flows from interfering with the resistance reading.
It is apparent from the foregoing description and discussion of the prior art that improvements in the art of temperature control of multiple section glass forming bushings are not only desirable but possible.